fix warnings/hints in nimdoc/tester.nim (#18083)

* fix warnings/hints in nimdoc/tester.nim

* improve err msg for nimdoc/tester.nim and change flag from fixup to nimTestsNimdocFixup

* address comment: put back quit instead of doAssert

nimdoc/testproject/testproject.nim

@@ -131,6 +131,7 @@ when true:
       # BUG: this currently this won't be run since not exported
       # but probably should
       doAssert false
+  if false: bazNonExported() # silence XDeclaredButNotUsed
 
   proc z17*() =
     # BUG: a comment before 1st doc comment currently doesn't prevent
@@ -212,7 +213,7 @@ when true: # tests RST inside comments
 
 when true: # multiline string litterals
   proc tripleStrLitTest*() =
-    runnableExamples:
+    runnableExamples("--hint:XDeclaredButNotUsed:off"):
       ## mullitline string litterals are tricky as their indentation can span
       ## below that of the runnableExamples
       let s1a = """
@@ -252,12 +253,12 @@ at indent 0
 
 when true: # methods; issue #14691
   type Moo = object
-  method method1*(self: Moo) =
+  method method1*(self: Moo) {.base.} =
     ## foo1
-  method method2*(self: Moo): int =
+  method method2*(self: Moo): int {.base.} =
     ## foo2
     result = 1
-  method method3*(self: Moo): int =
+  method method3*(self: Moo): int {.base.} =
     ## foo3
     1